Embryology Weeks 1-4

Front 1

blastomere

Back 1

individual cells of the morula (from zygote to morula, these cells are blastomeres)

week 1

Front 2

morula

Back 2

made of 16-32 blastomeres. same size as initial zygote. as it enters the uterine cavity, it becomes the blastocyst.

week 1

Front 3

blastocyst

Back 3

the morula becomes the blastocyst as it enters the uterina cavity. it develops 2 layers: the outer cell mass (trophoblast) and the inner cell mass (embryoblast). the inside of the blastocyst is the blastocyst cavity.

after the zona pellucida disappears, the trophoblast becomes an outer layer, the syncytiotrophoblast) and an inner layer (the cytotrophoblast).the embroblast develops a layer of cells on the surface called the hypoblast.

week 1

Front 4

zona pellucida

Back 4

protective covering of the zygote. covers the developing fertilized egg until the blastocyst reaches the uterine cavity. as this point it disappears, allowing the blastocyst to grow rapidly in size (prior to this, the zona pellucida inhibits growth in size).

week 1

Front 5

syncytiotrophoblast

Back 5

just prior to implantation, the trophoblast becomes two layers, the syncytiotrophoblast (outer layer) and the inner cytotrophoblast). this later invades the uterine wall and produces HCG (basis for pregnancy test).

week 1

Front 6

cytotrophoblast

Back 6

just prior to implantation, the trophoblast becomes two layers, the syncytiotrophoblast (outer layer) and the inner cytotrophoblast). this later provides new cells.

week 1

Front 7

hypoblast

Back 7

after the trophoblast becomes the outer syncytiotrophoblast and the inner cytotrophoblast and the blastocyst implants itself, the embryoblast develops a separate layer of cells around its surface, known as the hypoblast.

week 1

Front 8

first week of embryo

Back 8

zygote --> morula --> blastocyst --> implants into uterine wall

Front 9

epiblast

Back 9

during week 2, the blastocyst implants further into the uterine wall. the inner cell mass becomes the epiblast and the hypoblast, 2 distinct laters that touch each other.

Front 10

bilaminar germ disc

Back 10

aka the embryonic disc

this is where the epiblast and hypoblast meet. it is a disc like shape.

above the bilaminar disk is the epiblast later. this develops a new cavity, the amniotic cavity, which fills with amniotic fluid. this cavity is lined with specialized epiblast cells called amnioblasts.

below the bilaminar disc is the hypoblast layer. this layer develops a cavity, known as the exocoelomic cavity or primary yolk sac. cells lining the primary yolk sac produce a layer of loose connective tissue, called the extraembryonic mesoderm (sorrounds the yokk sac, amnion, and the bilaminar germ disc).

week 2

Front 11

amniotic cavity

Back 11

the amniotic cavity develops above the bilaminar germ disc, out of the epiblast. the amniotic cavity is filled with amniotic fluid and lines with specialized epiblast cells called amnioblast.

week 2

Front 12

amnioblasts

Back 12

specialized epiblast cells that line the amniotic cavity

week 2

Front 13

exocoelomic cavity

Back 13

aka the primary yolk sac.

develops below the bilaminar germ disc, out of the hypoblast layer. it is lined by cells that produce a loose layer of connective tissue on the outside of the yolk sac, called the extraembryonic mesoderm (surrounds yolk sac, bilaminar germ disc, and amnion).

week 2

Front 14

extraembryonic mesoderm

Back 14

a layer of loose connective tissues that surround the primary yolk sac, the bilaminar germ disc, and the amnion. it is produced by cells of the hypoblast layer, the line the primary yolk sac.

week 2

Front 15

extraembryonic coelom

Back 15

after the extraembryonic mesoderm forms, surrounding the yolk sac, bilaminar germ disc, and the amnion, vesicles appear in the mesoderm.

these vesicle coalesce to form the extraembryonic coelom.

the extraembryonic coelon separates the extraembryonic mesoderm into 2 layers:
-extraembryonic somatic mesoderm--applied to the trophoblast
-extraembryonic splanchic mesoderm--applied to the yolk sac

week 2

Front 16

extraembryonic somatic mesoderm

Back 16

the extraembryonic coelom separates the extraembryonic mesoderm into 2 layers:
-extraembryonic somatic mesoderm--applied to the trophoblast
-extraembryonic splanchic mesoderm--applied to the yolk sac

week 2

Front 17

extraembryonic splanchic mesoderm

Back 17

the extraembryonic coelom separates the extraembryonic mesoderm into 2 layers:
-extraembryonic somatic mesoderm--applied to the trophoblast
-extraembryonic splanchic mesoderm--applied to the yolk sac

week 2

Front 18

chorionic cavity

Back 18

after the extraembryonic membrane is separated into 2 laters by the extraembryonic coelom (the extraembryonic somatic mesoderm and the extraembryonic splanchnic mesoderm), the extraembryonic coelom is renamed the chorionic cavity. the growth of this cavity pinches off the primary yolk sac, forming the secondary yolk sac.

week 2

Front 19

secondary yolk sac

Back 19

the definitive yolk sac

formed where the emxtraembryonic coelom becomes the chorionic cavity. the primary yolk sac is pinched as the chorionic cavity pinches off a small piece of it.

the secondary yolk sac eventually becomes the gi tract and its derivatives.

week 2

Front 20

oropharyngeal membrane

Back 20

site of the future mouth.

forms from a section of the bilaminar disc, where a small circular area of hypoblast cells become attached to the overlying epiblast cells. opposite end of the disc from the cloacal membrane.

week 2

Front 21

cloacal membrane

Back 21

site of the future anus.

forms from a section of the bilaminar disc, where a small circular area of hypoblast cells become attached to the overlying epiblast cells. opposite end of the disc from the oropharyngeal membrane.

week 2